This invention relates to light shutters. In particular, this invention relates to electrooptic PLZT light shutters which might be used in a camera for example.
PLZT, which is lanthanum modified lead zirconate titanate, is a polycrystalline material, the structure of which is distorted by the application of an electric field in the material. This crystallographic distortion by an E-field alters the optical properties of the material causing what is known in the art as bi-refringence. A light wave incident upon the PLZT will split into two components. The light wave component perpendicular to the electric field and that which is parallel to the electric field travel at different velocities because the applied electric field changes the index of refraction of the material, depending upon the direction of propagation with respect to the orientation of the E-field. The two light wave components recombine upon exiting the PLZT crystal producing an effective rotation of the polarization of the light wave by the PLZT. With no field applied to the PLZT, light passes through the PLZT unaltered.
The ability of PLZT to rotate the polarization of a light wave has been used to provide a solid state electronic camera shutter when the PLZT is used in combination with at least one other discrete light polarizer. If a light polarizer oriented to polarize light at an angle of forty-five degrees to the E-field is optically coupled to send light to a PLZT element, which when it is subjected to an E-field rotates light ninety degrees, and a second polarizer receiving light from the PLZT is oriented to align with the rotated polarization vector, the light will pass through the second polarizer. When the E-field is removed, the PLZT will no longer rotate the light wave, passing light orthogonal to the orientation of the second polarizer which will then block the light.
A PLZT-based single lens reflex (SLR) shutter is shown in FIG. 1. A light beam from a light source is split into two components by a beam splitter (16) so that a user (32) can view the light source (by means of the reflector which is typically a prism) that will strike the target film (28) when the "shutter" is opened. Light (12) will strike the target film (28) if the light from the PLZT filter (24) is polarized by the PLZT to align with the orientation of the second polarizer (34). The first polarizer (22) orients the light (12) to a single direction so that the PLZT rotates the light to either align with or cross with the orientation of a second polarizer (34). Light from the PLZT (24) will either align with the second polarizer (34) or will be blocked by the polarizer (34) depending upon whether the E-field is applied or removed. (The rotation of polarization by the PLZT actually changes smoothly with the strength of the E-field. Increasing and decreasing field strength increases and decreases rotation substantially proportionately.) Light through the PLZT might have to be rotated in the PLZT or un-rotated depending upon the orientation of the light entering the PLZT, which is determined by the orientation of the first polarizer (22).
An electronic shutter is realized by the orientation of the polarizers (22 and 34) with respect to each other and the state of the E-field on the PLZT. If the first and second polarizers are aligned together, light will pass through both polarizers to the target (28) unless the light is rotated by the PLZT. If the first and second polarizers are orthogonal to each other, light will not pass through the second polarizer unless the PLZT rotates it.
In FIG. 1 a diffraction mirror (16) splits incoming light into two components (18 and 20) so that one component (18) is reflected to a user (32) permitting the user (32) to view what the film (28) will be exposed to. A significant problem with SLR shutter systems including prior art PLZT based shutter systems, is the flare produced by the electrodes on the PLZT surface as well as the flare produced by the diffraction mirror (16). This flare is light reflection produced by reflective surfaces (on the splitter (16), electrodes (26) and other surfaces) which blurs or distorts the image received at the film (28).
Prior art SLR shutters including PLZT based shutters have attempted to reduce the amount of flare by coating reflective surfaces with dark colored, light absorbing coatings. PLZT-based shutters in particular have required coated electrodes on the PLZT element (24) to reduce flare. These coated electrodes have been painted black and the spacing and reflecting bar width of the diffraction mirror coatings being adjusted to reduce flare.
Prior art attempts to reduce the amount of flare are expensive and require precision manufacturing techniques. A PLZT based shutter system which reduces optical flare would be an improvement over the prior art.